By Robin Lucas, CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=12936477
If you describe someone as treacherous, you mean that they are likely to betray you and cannot be trusted.
If you say that something is treacherous, you mean that it is very dangerous and unpredictable.
The current of the river is fast flowing and treacherous.
They made the treacherous journey across stormy seas in rotten boats.
